Marine Dadabacteria exhibit genome streamlining and phototrophy-driven niche partitioning

The remineralization of organic material via heterotrophy in the marine environment is performed by a diverse and varied group of microorganisms that can specialize in the type of organic material degraded and the niche they occupy. The marine Dadabacteria are cosmopolitan in the marine environment and belong to a candidate phylum for which there has not been a comprehensive assessment of the available genomic data to date. Here in, we assess the functional potential of the oligotrophic, marine Dadabacteria in comparison to terrestrial, coastal, and subsurface members of the phylum. Our analysis reveals that the marine Dadabacteria have undergone a genome streamlining event, reducing their genome size and the nitrogen content of their DNA and predicted proteome, relative to their terrestrial counterparts. Collectively, the Dadabacteria have the potential to degrade microbial particulate organic matter, specifically peptidoglycan and phospholipids. The marine Dadabacteria belong to two clades with distinct ecological niches in global metagenomic data: a shallow clade with the potential for photoheterotrophy through the use of proteorhodopsin, present predominantly in surface waters up to 100m depth; and a deep clade lacking the potential for photoheterotrophy that is more abundant in the deep photic zone.